The X-ray CT apparatus emits X-rays while rotating an X-ray source and an X-ray detector, which is disposed so as to face the X-ray source, around an object, detects X-rays transmitted through the object over the entire circumference, and performs image reconstruction on the basis of the obtained projection data to obtain a tomographic image. In this image reconstruction, it is a requirement that the object or the organ in the object be stationary. If the object or the organ moves while collecting projection data, the target is not correctly imaged at the time of image reconstruction. As a result, a blurred image (motion artifacts) is obtained.
In particular, since the heart is an organ that is constantly beating, time to collect projection data, that is, time resolution needs to be improved in order to obtain an image of a stationary heart. Therefore, in cardiac imaging using an X-ray CT apparatus, an ECG-gated reconstruction method is performed in which an electrocardiograph is mounted on an object, projection data of the phase at which the motion of the heart is small is collected using the electrocardiogram information of the object, and image reconstruction is performed to obtain an image of the relatively stationary heart.
An imaging method in cardiac imaging using an X-ray CT apparatus is largely divided into two methods of the Retrospective Gating method and the Prospective Triggering method.
The Retrospective Gating method is a method in which a helical scan is performed while collecting electrocardiogram information of the object, only projection data of the phase at which the motion is small is collected from the obtained projection data using the electrocardiogram information, and image reconstruction is performed as described previously. In this method, since X-rays are continuously emitted to perform a helical scan, it is possible to generate an image of an arbitrary phase. However, since it is necessary to collect projection data of the same phase from a plurality of cardiac beats, the moving speed of the table is reduced. Accordingly, the amount of exposure is several times the normal helical scan.
The Prospective Triggering method is a method in which the electrocardiogram information of an object is monitored and X-rays are emitted only after a time, at which a specific phase set in advance is obtained, from the detection of an R wave, for example, to perform an axial scan. In this case, since X-rays are emitted only in a specific phase without emitting X-rays continuously, the amount of exposure is very small compared with the Retrospective Gating method (PTL 1).
Although each method has advantages and disadvantages, it is desirable to perform imaging using the Prospective Triggering method with less exposure when a burden on the object is taken into consideration.